Quick iPhone 4(Verizon) question

Bought a iPhone 4(Verizon model) from a guy i work with a few months ago. My brothers phone (first Droid) is dying and needed a phone. I use the iPhone 4 as an iPod since i do not have verizon and thought i would lend it to him to use until his upgrade. So he took the phone to verizon and they plugged it in to check it and transfer his stuff over and they said that there is a balance due attached to the iPhone number. They said the iPhone cannot be used until the balance is paid or another phone is activated with its number, and sent my brother on his way. I don't understand why they can't just change the number over so then he can use it. Has anyone ran into this problem? And how did you go about getting the phone activated? Thanks for any helpful responses.

Also, I have sprint and since the iPhone 4 will be on sprints network, if verizon still won't activate the iPhone do you think sprint possibly would?

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